Factors Affecting the Measurement of Energy Expenditure during Energy Balance Studies in Preterm Infants

Our objective was to examine factors that affect the accuracy of energy expenditure measurements (EE), when using flow-through indirect calorimetry (IC), to determine the minimum length of time needed to measure 24- and 48-h EE and to compare cross-over and parallel designs as methods of investigation during energy balance collections (EB) in preterm infants. A baby doll manikin was used to determine equilibration times and to compare VCO2 and VO2 as measured by flow meter and indirect calorimetry under different study conditions, one of which simulated an EB. “Continuous” EE was measured to determine the minimum length of time needed to accurately reflect 24- and 48-h EE and to compare parallel and cross-over studies as methods of study design in a group of “normal” enterally fed preterm infants. The mean (±SD) errors between flow meter and indirect calorimetry determinations for VCO2 and VO2 were −1.9 ± 2.5 and −1.8 ± 4.3% under conditions that simulated an EB. Cumulative 6-h EE accurately predicted 24- and 48-h EE. Expressed in absolute terms (kcal/d), EE did not change on a day-to-day basis but did increase over the 2-wk study period. Expressed on body weight basis (kcal/kg/d), EE did not change on a day-to-day or week-to-week basis. The variance in EE due to biologic variability;i.e. the parallel design, was ∼6 times greater than that due to age, weight, and weight gain;i.e. the cross-over design. Indirect calorimetry, therefore, accurately measures EE in conditions simulating an energy balance collection. Six-hour EE determinations are valid estimate of EE during a 48-h balance collection, while cross-over studies may be the preferred method of study design during short-term studies of EE in preterm infants.